[O I] 6300 Å emission in Herbig Ae/Be systems: Signature of Keplerian rotation

We present high spectral-resolution optical spectra of 49 Herbig Ae/Be stars in a search for the [OI] 6300 A line. The vast majority of the stars in our sample show narrow (FWHM < 100 km s -1 ) emission lines, centered on the stellar radial velocity. In only three sources is the feature much broader (∼400 km s -1 ), and strongly blueshifted (-200 km s -1 ) compared to the stellar radial velocity. Some stars in our sample show double-peaked line profiles, with peak-to-peak separations of ∼10 km s -1 . The presence and strength of the [OI] line emission appears to be correlated with the far-infrared energy distribution of each source: stars with a strong excess at 60 μm have in general stronger [OI] emission than stars with weaker 60 μm excesses. We interpret these narrow [OI] 6300 A line profiles as arising in the surface layers of the protoplanetary disks surrounding Herbig Ae/Be stars. A simple model for [OI] 6300 A line emission due to the photodissociation of OH molecules shows that our results are in quantitative agreement with that expected from the emission of a flared disk if the fractional OH abundance is ∼5 × 10 -7 .